A simplified analytical model was developed and implemented to simulate changes in turbine tip clearance during the operation of a commercial gas turbine engine. The clearance model is an enabling technology for the fast-response active turbine tip-clearance control being developed by the NASA Glenn Research Center under the Ultra-Efficient Engine Technology Project.
Changes in tip clearance during a notional mission profile and turbine stage cross section.
Long description.
As shown in the preceding figure, fast-response active turbine tip clearance control is an attempt to improve engine efficiency and reduce peak exhaust gas temperatures by manipulating both transient and steady-state clearances during engine operation. A fast-acting clearance control system would provide minimal clearance throughout the flight profile as shown by the dashed line. Existing clearance control systems are based on slow thermal processes that cannot mitigate reacceleration transients. Recent studies equate a 10-mil reduction in turbine tip clearance to a reduction in engine exhaust gas temperature of up to 10 °C and an increase in turbine efficiency of up to 1 percent. Reductions of 10 to 30 mils at cruise are expected with a fast-acting clearance control.
One of the critical technology gaps in the development of a fast-acting clearance control system is a dynamic model of the clearance phenomena. Although models of the clearance dynamics are alluded to in the open literature, they tend to be proprietary in nature and, as a result, are not published in the open literature. In order to address this gap, Glenn, in partnership with the University of Texas Pan American, is developing a generic model to simulate the clearance dynamics. A first-principles approach is used to estimate the clearance by modelling the thermal and mechanical effects of engine operating conditions on the radial deformation of the turbine subcomponents--blade, rotor, and shroud--at each point in time. The model uses engine speed, temperatures, and pressures with thermal and mechanical constants to compute the deformation of the turbine subcomponents at each point in time. The mechanical and thermal deformation of each subcomponent is then added to the cold-build radius (or length for the blade) to obtain the deformed radius. The clearance then becomes the difference between the deformed radius of the shroud and that of the rotor plus the blade.
Partnership researchers implemented the model in a graphical simulation environment, using the speeds, temperatures, and pressures from the simulation of an engine representative of modern fighter aircraft as input. The following graphs show results from the simulation. The pinch point is clearly visible on the lower axis. Ongoing efforts are focused on improving and validating the model.
Results from dynamic simulation of the turbine clearance model. G/I, ground idle.
Long description.
Find out more about this research: http://www.grc.nasa.gov/WWW/cdtb/
Melcher, K.J.; and Kypuros, J.A.: Toward a Fast-Response Active Turbine Tip Clearance Control, Proceedings of the XVI International Symposium on Air Breathing Engines, Cleveland, OH, Aug. 31-Sep. 5, 2003, ISABE 2003-1102 (NASA/TM-2003-212627), 2003. http://gltrs.grc.nasa.gov/cgi-bin/GLTRS/browse.pl?2003/TM-2003-212627-REV1.html
Kypuros, Javier A.; and Melcher, Kevin J.: A Reduced Model for Prediction of Thermal and Rotational Effects on Turbine Tip Clearance. NASA/TM-2003-212226, 2003. http://gltrs.grc.nasa.gov/cgi-bin/GLTRS/browse.pl?2003/TM-2003-212226.html
Lattime, Scott B.; and Steinetz, Bruce M.: Turbine Engine Clearance Control Systems: Current Practices and Future Directions. AIAA Paper 2002-3790 (NASA/TM-2002-211794), 2002. http://gltrs.grc.nasa.gov/cgi-bin/GLTRS/browse.pl?2002/TM-2002-211794.html
Glenn contact: Kevin J. Melcher, 216-433-3743, Kevin.J.Melcher@nasa.gov
Author: Kevin J. Melcher
Headquarters program office: OAT
Programs/Projects: Propulsion and Power, UEET
Last updated: January 20, 2005
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